SPID#: 59 In order to study the effects of altering the prenatal hormonal environment on behavioral development in males and females we produced time-mated pregnancies in a two large social group of 75 rhesus monkeys each. Five time times weekly 90 min of behavioral observations were collected where all sexual interactions between any male and the groups' females was recorded using computerized event-sequential methodology. Twenty to 24 days after the last day of mating pregnancy was verified using ultrasonic visulaization. This technique produced 41 verified pregnancies. 12 pregnancies received 30mg/kg of the anti-androgen flutamide (FL) dissolved in DMSO IM twice daily from gestational days 40-70, five pregnancies received FL from gestational days 110-140. 12 pregnant females received weekly IM injections of 20 mg of testosterone enanthate (TE) disslved in oil, and six pregnant females received TE treatment from gestational days 110-140. An additional six females received twice-daily control vehicle injections. The injection schedule had only just begun at the end of the reporting period thus the outcome awaits the current birth season. It is anticipated that fetal females exposed to TE during days 40-70 of gestation will be extensively anatomically virilized, but show little modification of sexually dimporphic behavior. Late TE-treated females, in contrast, will show little physical virillization, but are likely to show extensive behavioral masculinization. Fetal males exposed to FL early in pregnancy should not be anatomically masculinized, but will be masculinized behaviorally. Late FL treatment should have the opposite effects on anatomy and behavior in fetal males.